Performance of radio receivers are highly dependent on the selectivity of the receiver front end. Front-end selectivity is generally provided by a pass-band filter. Pass band filters come in variety of structural and topological arrangements. Most pass-band filters include a plurality of reactively coupled resonators which are resonant at a center frequency. Generally, the resonators are coupled to each other by reactive components, such as capacitors, inductors or combination thereof. A number of factors, including the type and number of resonators in the filter topology, determine the selectivity of a filter. Depending on the application, the filter topology may include any number of quarter-wave or half-wave resonators, or a combination of them.
Front-end filters may have fixed bandwidths or they may have tunable bandwidths. Filters having fixed bandwidths are usually used in narrow band receivers where the operational frequency is limited to a narrow range. On the other hand, filters having tunable bandwidth may be used in wideband receivers where the range of operational frequency is substantially large and covers a wider bandwidth. The tunable filters are particularly suitable in reducing manufacturing cost of receiver units because they allow a single receiver front end circuit to cover an entire allocated frequency band, thus eliminating the need for designing multiple front-end circuit boards for covering a number of sub-bands within the frequency band.
Most tunable filters are varactor tuned. Each resonator includes a tuning element comprising one or more varactors which in response to a control voltage provide the tuning mechanism for the tunable filter. By varying the control voltage, the filter may be tuned to provides a narrow bandwidth for a desired frequency of operation. Each resonator includes a tuning element, such as a single varactor a pair of back-to-back varactors such that upon simultaneous application of the control voltage, the center frequencies of the resonators are varied. In this way the tuning elements receive identical control voltages, and as these control voltages vary, the filter is tuned for a desired bandwidth within a tuning range.
An important consideration in designing tunable filters is maintaining a constant bandwidth throughout the tuning range. Conventional approaches, however, cannot maintain such a constant bandwidth. Thus, at certain frequencies, within the tuning range, the bandwidth of the filter varies as the control voltage applied to the varactors varies. Another important factor in tunable filter design is providing a substantially high image rejection. Conventional resonator coupling techniques which utilize a capacitor or a combination of capacitors and inductor has been found to be a major contributor to the occurrence of non-constant bandwidth over the tuning range where the bandwidth becomes wider and wider when the filter is tuned from a low to a high frequency. As a result, near the high end of the RF, image rejection becomes very poor and tuning becomes meaningless.
A partial solution has been found, using purely inductive coupling between the resonators. As such, a single coupling inductor makes the bandwidth of the filter independent from the resonator frequency, thereby the bandwidth is maintained constant over the tuning range since. However, such purely inductive coupling still does not address or improve the image rejection performance of the filter. Therefore, there still exists a need for a tunable filter which maintains a substantially constant narrow bandwidth over the entire tuning range while providing a substantially superior image rejection performance.